Giuseppe Barillaro

Electrochemical etching in HF solution for silicon micromachining

Abstract Electrochemical etching of silicon in hydrofluoric acid (HF) solution is employed as a micromachining technique. It is demonstrated that the commonly accepted geometric constraints on the shape of electrochemically etched silicon structures can be significantly relaxed. Several new structures etched on the same n-doped silicon wafer are reported. The fabricated structures include wall arrays, hole arrays, meander-shaped structures, spiral-like walls, microtubes, micropillars, microtips and more. A simple model for the electrochemical etch process, which describes the effect of the dimension of the initial seed, the current density, and also the KOH etching time of the initial pattern on the final geometries, is detailed.

APSFET: a new, porous silicon-based gas sensing device

In this paper, a new sensing device based on a FET structure having a PoSi layer as sensing material, namely adsorption porous silicon-based FET (APSFET), is proposed. The sensing mechanism is based on an gas-induced conduction channel in the crystalline silicon under the sensing layer, a new approach with respect to previously reported PoSi sensors. The fabrication process is based on a standard silicon process. In this work, the fabrication process along with an electrical characterization of the device in presence of different organic vapors (alcohols and acids) is presented and discussed.

Analysis, simulation and relative performances of two kinds of serpentine springs

A complete set of approximate, closed-form expressions, obtained with small displacements theory, for the spring constants of two kinds of serpentine springs (classic and rotated) is presented. The expressions are proposed as a tool for rapid design of microelectromechanical structures. The two designs are also studied numerically using the finite element method (FEM): analytical calculations and FEM simulations are compared. A comparison between the performances of the two designs is also briefly presented.

Microneedles for Transdermal Biosensing: Current Picture and Future Direction

A novel trend is rapidly emerging in the use of microneedles, which are a miniaturized replica of hypodermic needles with length-scales of hundreds of micrometers, aimed at the transdermal biosensing of analytes of clinical interest, e.g., glucose, biomarkers, and others. Transdermal biosensing via microneedles offers remarkable opportunities for moving biosensing technologies and biochips from research laboratories to real-field applications, and envisages easy-to-use point-of-care microdevices with pain-free, minimally invasive, and minimal-training features that are very attractive for both developed and emerging countries. In addition to this, microneedles for transdermal biosensing offer a unique possibility for the development of biochips provided with end-effectors for their interaction with the biological system under investigation. Direct and efficient collection of the biological sample to be analyzed will then become feasible in situ at the same length-scale of the other biochip components by minimally trained personnel and in a minimally invasive fashion. This would eliminate the need for blood extraction using hypodermic needles and reduce, in turn, related problems, such as patient infections, sample contaminations, analysis artifacts, etc. The aim here is to provide a thorough and critical analysis of state-of-the-art developments in this novel research trend, and to bridge the gap between microneedles and biosensors.

Dimensional Constraints on High Aspect Ratio Silicon Microstructures Fabricated by HF Photoelectrochemical Etching

The fabrication of macropores in crystalline silicon by photoelectrochemical etching in a hydrofluoric acid electrolyte is investigated. It is shown that the dimensional constraints on the pore diameters, which, in previous literature, are considered to depend on substrate doping, can be significantly relaxed. We show that it is possible to fabricate arrays of square section macropores with sides ranging from 2 to 15 mm using the same n-doped ~2.4-4 V cm! silicon substrate. Moreover, we demonstrate that macropore arrays with pitch variation up to 100% ~8 and 16 mm! can be simultaneously grown on the same silicon sample. The same process is used to fabricate arrays of silicon walls with different spacing and pitch as well. A simple model, based on the coalescence in a single pore of multiple stable pores, is proposed to explain the experimental data.

Optofluidic microsystems with integrated vertical one-dimensional photonic crystals for chemical analysis

In this work, we report all-silicon, integrated optofluidic microsystems (OFMs) fabricated by electrochemical micromachining (ECM) technology, in which high aspect-ratio (HAR) photonic crystal (PhC) devices (i.e. micromirrors, optical cavities) are integrated by one-etching-step, together with microfluidic reservoirs/channels, for the infiltration of liquids in the PhC air gaps, and with fiber grooves for alignment/positioning of readout optical fibers in front of the PhC, on the same silicon die. This has not previously been reported in the literature, and opens up new ground in, though not limited to, the optofluidics field, due to the low-cost and high-flexibility of the ECM technology that allows optofluidic microsystem fabrication to be performed in any lab. Optofluidic characterization of PhC-OFMs by both capillary-action and pressure-driven operations is carried out through the measurement of the reflectivity spectra of HAR-PhCs upon injection of liquids featuring different refractive index values in the HAR-PhC air gaps, by using readout optical fibers positioned in the on-chip fiber grooves. High sensitivity and good limit of detection of PhC-OFMs are obtained for both capillary-action and pressure-driven operations. A best sensitivity value of 670 nm/RIU and a worst-case limit of detection of the order of 10(-3) RIU are measured, the former being comparable to state-of-the-art integrated refractive index sensors and the latter being limited by constraints of the experimental setup. The proof of concept about the biosensing potential of PhC-OFMs is given by successfully carrying out a sandwich assay based on antigen-antibody interactions for the detection of the C-reactive protein (CRP) at a concentration value of 10 mg L(-1), which represents the boundary level between physiological and pathological conditions.

Optical Characterization of High-Order 1-D Silicon Photonic Crystals

In this paper, we present numerical and experimental results on the spectral reflectivity of hybrid, high-order (up to 22nd) 1-D silicon photonic crystals (PCs) in the near-infrared region (wavelength range 1- 1.7 mum). Mechanically robust, vertical 1-D PCs with high aspect ratio and spatial period of 8 mum were fabricated by electrochemical micromachining of silicon, and tested in reflection with an improved optical setup, incorporating standard telecommunication single-mode optical fibers and a lensed fiber pigtail. A detailed theoretical, numerical analysis was performed to assess the effects of both non-idealities of the structures under test and constraints of the optical setup, on the spectral reflectivity. Experimental data were found in very good agreement with theoretical calculations, performed by using the characteristic matrix method, keeping into account an in-plane porosity variation for 1-D PCs, due to surface roughness of silicon walls, and the limited resolution bandwidth of the spectrum analyzer. Best optical performances, measured on the fabricated 1-D PC mirrors, consist of optical losses less than 0.8 dB in a bandgap around 1.5 mum and a -35 dB reflectivity minimum at a bandgap edge.

Self-powered microneedle-based biosensors for pain-free high-accuracy measurement of glycaemia in interstitial fluid.

In this work a novel self-powered microneedle-based transdermal biosensor for pain-free high-accuracy real-time measurement of glycaemia in interstitial fluid (ISF) is reported. The proposed transdermal biosensor makes use of an array of silicon-dioxide hollow microneedles that are about one order of magnitude both smaller (borehole down to 4µm) and more densely-packed (up to 1×10(6)needles/cm(2)) than state-of-the-art microneedles used for biosensing so far. This allows self-powered (i.e. pump-free) uptake of ISF to be carried out with high efficacy and reliability in a few seconds (uptake rate up to 1µl/s) by exploiting capillarity in the microneedles. By coupling the microneedles operating under capillary-action with an enzymatic glucose biosensor integrated on the back-side of the needle-chip, glucose measurements are performed with high accuracy (±20% of the actual glucose level for 96% of measures) and reproducibility (coefficient of variation 8.56%) in real-time (30s) over the range 0-630mg/dl, thus significantly improving microneedle-based biosensor performance with respect to the state-of-the-art.

Silicon micromachined periodic structures for optical applications at λ=1.55μm

In this letter, the authors report the design, fabrication, and characterization of a silicon micromachined periodic structure for optical applications at λc=1.55μm. The microstructure, which can be envisioned as a one-dimensional photonic crystal, is composed of a periodic array of 1-μm-thick silicon walls and 2-μm-wide air gaps, each one corresponding to a different odd number of quarter wavelength at λc (hybrid quarter wavelength). The fabrication is based on the electrochemical etching of silicon, yielding parallel trenches with depths up to 100μm. Preliminary reflectivity measurements show the presence of a band gap at λc=1.55μm, as theoretically expected.

Low-concentration NO/sub 2/ detection with an adsorption porous silicon FET

Adsorption porous silicon FET (APSFET) is a porous silicon (PS)-based device constituted of a FET structure with a porous adsorbing layer between drain and source. Adsorbed gas molecules in the porous layer induce an inverted channel in the crystalline silicon under the PS itself. The mobile charge per unit area in the channel depends on the molecular gas concentrations in the sensing layer so that adsorbed gas molecules play a role similar to the charge on the gate of a FET. In this work, NO/sub 2/ detection by using the APSFET is demonstrated for the first time. NO/sub 2/ concentration as low as 100 ppb was detected. Devices with both as-grown and oxidized PS layers were fabricated and compared in order to investigate the effect of a low-temperature thermal oxidation on the electrical performances of the sensor. Nonoxidized sensors show a high sensitivity only for fresh devices, which reduces with the aging of the sample. Oxidation of the PS layer improves the electrical performance of sensors, in terms of stability, recovery time, and interference with the relative humidity level, keeping the high sensitivity to nitrogen dioxide.